e: Two Different Worlds

 Weird things happened in the outer Solar System when it was first a'borning. The ice-giants, Uranus and Neptune, are the two outermost major planets of our Sun's family, and in size, bulk, composition, and great distance from our Star, they are very much alike. Both distant worlds are clearly different from the quartet of small rocky inner planets--Mercury, Venus, Earth, and Mars--as well as from the duo of gas-giant planets, Jupiter and Saturn. Ice giants are planets that contain elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. Although the two planets should be almost identical twins, they are not. In February 2020, a team of planetary scientists from the University of Zurich in Bern, Switzerland, told the press that they believe they have discovered why.

"There are... striking differences between the two planets that require explanation," commented Dr. Christian Reinhardt in a February 2020 PlanetS Press Release. Dr. Reinhardt studied Uranus and Neptune with Dr. Alice Chau, Dr. Joachim Stadel and Dr. Ravit Helled, who are all PlanetS members working at the University of Zurich, Institute for Computational Science.

Dr. Stadel commented in the same PlanetS Press Release that one of the striking differences between the two planets is that "Uranus and its major satellites are tilted about 97 degrees into the solar plane and the planet effectively rotates retrograde with respect to the Sun".

In addition, the satellite systems of the distant duo are different. Uranus's major satellites are on regular orbits and tilted with the planet, which suggests that they formed from a disk, similar to Earth's Moon. In contrast, Triton--Neptune's largest moon--is very inclined, and is therefore considered to be a captured object. Triton also displays important similarities to the distant ice-dwarf planet, Pluto, which suggests that the two may have been born in the same region--the Kuiper belt that is situated beyond Neptune's orbit, and is the frigid, dimly lit home of myriad comet nuclei, small minor planets, and other frozen bodies. Planetary scientists predict that in the future Triton's orbit will decay to the point that it will crash down into its adopted parent-planet.

In addition to other differences, Uranus and Neptune may also differ in respect to heat fluxes and internal structure.

Ice-Giants

In astrophysics and planetary science the term "ices" refers to volatile chemical compounds that possess freezing points above around 100 K. These compounds include water, ammonia, and methane, with freezing points of 273 K, 195 K, and 91 K, respectively. Back in the 1990s, scientists first came to the realization that Uranus and Neptune are a distinct class of giant planet, very different from the two other giant denizens of our Sun's family, Jupiter and Saturn. The constituent compounds of the duo of ice giants were solids when they were primarily incorporated into the two planets during their ancient formation--either directly in the form of ices or encased in water ice. Currently, very little of the water in Uranus and Neptune remains in the form of ices. Instead, water mostly exists as supercritical fluid at the temperatures and pressures within them.

The overall composition of the duo of ice giants is only about 20% hydrogen and helium in mass. This differs significantly from the composition of our Solar System's two gas-giants. Jupiter and Saturn are both more than 90% hydrogen and helium in mass.

Modelling the formation history of the terrestrial and gas-giant planets inhabiting our Solar System is relatively straightforward. The quartet of terrestrial planets are generally thought to have been born as the result of collisions and mergers of planetesimals within the protoplanetary accretion disk. The accretion disk surrounding our newborn Sun was composed of gas and dust, and the extremely fine dust motes possessed a natural "stickiness". The tiny particles of dust collided into one another and merged to form bodies that gradually grew in size--from pebble size, to boulder size, to moon size, and ultimately to planet size. The rocky and metallic planetesimals of the primordial Solar System served as the "seeds" from which the terrestrial planets grew. Asteroids are the lingering relics of this once-abundant population of rocky and metallic planetsimals that ultimately became Mercury, Venus, Earth, and Mars.